Cooling systems for both industrial plants and utilities are subject to fouling by macroinvertebrates (i.e., mollusks, barnacles, bryozoans, sponges, tunicates, hydroids, annelids) whether the system is using cooling water on a once-through basis or of the recirculating type. Once-through systems operate by drawing cooling water through the process to be cooled on a one-time basis with a short residence time (usually minutes to hours) and discharging the water directly to a receiving body whereas, recirculation cooling systems require the addition of only a fraction of the system volume as makeup water. Additionally, service water systems such as waste, safety and auxiliary cooling which are often part of these cooling systems are also quite vulnerable to macroinvertebrate fouling, primarily because they do not run continuously, and the conduits are of a small diameter.
The extent and type of macroinvertebrate fouling will depend upon many factors such as the source of the cooling water, the season, the water temperature, the growth rate of the macroinvertebrate, and the linear velocity of the cooling water. Further, because of the large quantities of cooling water used, the locality of the plant will dictate the water's source. A fresh water cooling system will be drawing from a river, a lake, or a well, whereas plants situated along coastal areas will most likely utilize brackish or marine water for their cooling purposes.
Both once-through and recirculating types of cooling water are treated prior to entering the system by screening to remove objects which are large enough that they could damage pumps and heat exchange equipment. This screening does not, however, prevent the passage of early, microscopic life stage or larval stages of the macroinvertebrates, which are the precursors to fouling as growth conditions are usually favorable within these systems. These early life stages of the macroinvertebrates will settle out or attach in low flow areas within the cooling system and grow and accumulate to a fouling size.
For example, mollusks are common macroinvertebrates which can cause fouling problems to marine and fresh water cooling systems. Macrofouling by mollusks, like other groups of fouling mcaroinvertebrates-barnacles, bryozoans, sponges, hydroids, tunicates and annelids-is initiated by the settlement or attachment of larval and/or juvenile stages that are easily entrained into cooling water systems. Fouling caused by the settlement, attachment and/or biogrowth of the macroinvertebrates in the cooling systems and associated service water systems of the industrial plants and utilities which utilize large quantities of water is a major problem causing a variety of deleterious effects to the structure, operation and safety of these systems.
As indicated in the U.S. Nuclear Regulatory Commission 1984 Report entitled "Bivalve Fouling of Nuclear Power Plant Service-Water Systems", the safe operation of a nuclear power plant is a concern because of fouling caused by the Asiatic clam (Corbicula fluminea), the blue mussel (Mytilus edulis) and the American oyster (Crassostrea virginica). This report describes the correlations between the biology of these bivalve mollusks and the design and operation of power plants that allow bivalves to enter and reside within their cooling water systems.
One of the macroinvertebrates controlled by the method of this invention is the mollusk Asiatic clam, Corbicula spp. As indicated in the article entitled "Freshwater Macrofouling and Control with Emphasis on Corbicula" in the December 1983 Proceedings of the Electric Power Research Institute (EPRI), the Asiatic clam has caused significant incidents of macrofouling to fresh water cooling systems of power plants. Another freshwater mollusk, the Zebra mussel (Dreissena polymorph) causes fouling problems to cooling systems in a similar manner as the Asiatic clam. both Dreissena polymorph, causes fouling problems to cooling systems in a similar manner as the Asiatic clam. Both Dreissena and Corbicula have free floating planktonic veliger larvae which allow easy penetration into cooling systems. Similar macrofouling problems plague cooling systems using estuarine, brakish, or marine waters, but with different species of macroinvertebrates.
Fouling control of macroinvertebrates, such as mollusks has been attempted using physical/mechanical and chemical techniques (see, e.g., U.S. Pat. No. 4,328,638), but no truly foolproof combination has been developed.
Chlorine, a commonly used biofouling inhibiting agent, has several limitations with respect to treatment to control macroinvertebrates. Chlorine is very toxic to microorganisms and readily kills them at 1 or 2 mg/liter levels; however, mollusks can survive for a considerable period of time in water containing a much higher level of chlorine because of their anatomic and physiological development relative to microorganisms. Microorganisms must accept the environment they find themselves in and live or die depending upon the nature of the environment. On the other hand, higher animals such as mollusks, and other macroinvertebrates when they find themselves in an environment that is inhospitable, can either more or utilize defense systems to exclude the hostile environment. For example, bivalve mollusks can close their shells to exclude the hostile environment. Bivalve mollusks have very sensitive chemosensors in the mantle lining the edge of their shells and, even when their shells are tightly closed, they can continuously sample the environment to determine when it is safe to open up their shells and start siphoning again. A mollusk immersed in chlorine containing water so that its shell is bleached while will open up after the chlorine level drops and resume its life. As a result, biocides that are sensed by the bivalve mollusk's chemoreceptor organs as life threatening are not effective simply because the mollusk will close its shell until the threat passes. Mollusks can remain closed for days and still live and resume normal activity. For these reasons prolonged exposure to chlorine is required to achieve efficacy. Other limitations of chlorine treatment include the chlorine demand of the cooling water which reduces the potency of chlorine, and the strict environmental regulations being imposed which act to severely limit the discharge of chlorine residues, and in some cases seek to eliminate the use of chlorine entirely.
In addition to chlorine, U.S. Pat. Nos. 4,462,914 and 5,192,451 disclose the use of a high density cationic polymer to control Asiatic clams, Corbicula and Zebra Mussels, respectively. While the polymer appears to be efficacious toward these two mollusks after a six day exposure period, it suffers from some of the same drawbacks as chlorine.
The above-mentioned concerns over the potential environmental impact of biocides is well described by the following excerpt from the December 1983 Proceedings of the Electric Power Research Institute: "Chemical controls have an inherent liability. What can kill inside the power plant may also impact the receiving water body; chemical toxicants are not specific. The perfect chemical would be stable enough to be effective inside the plant, but become non-toxic, via chemical reaction or decay, before or as it entered the receiving water body. So far, no chemical meets these specifications: chlorine and bisulfate/sulfide, which have actually been used in an attempt to control macroinvertebrate fouling, have not been effective alone, or have been successful only under limited conditions. Such a chemical may not exist, but scheduling of application of a chemical at the beginning of scheduled outages may offer a less stringent alternative, because of the possibility of extending holdup times."
U.S. Pat. No. 4,561,983 discloses the use of nitrostyrene compound to control the fouling potential of mollusks. U.S. Pat. No. 4,579,665 discloses the use of a nitrostyrene compound and an alkyl thiocyanate compound to control mollusk fouling potential.
U.S. Pat. No. 4,816,163 discloses a method for controlling the fouling potential of macroinvertebraes, especially mollusks, in an aqueous system which comprises adding to the system an effective controlling amount of a water-soluble alkyl guanidine salt. U.S. Pat. No. 4,857,209 discloses a method for controlling the fouling potential of macroinvertebraes, especially mollusks, in an aqueous system which comprises adding to the system an effective controlling amount of a water-soluble quaternary ammonium salt with detergent properties.